This invention relates to modifying the inside bottom surface of a continuous casting tundish to mitigate molten metal turbulence thereby improving metal cleanliness. The invention includes an elongated tundish having a refractory ramp extending in a lateral direction and mounted to the inside bottom surface of the tundish. The ramp has first and second surfaces with the first surface inclined downwardly in a direction toward a tundish outlet and the second surface having a substantially vertical face. Inclusions caused by entrained gas bubbles and slag in a continuously cast slab, especially steel slabs, are reduced.
It is known to modify the inside bottom surface of a tundish to include means for reducing turbulence, gas bubble entrainment, slag entrainment and removing oxide inclusions from molten metal during continuous casting. For example, it is known to mount one or more dams to the inside bottom surface of a tundish with the dams extending in a transverse direction relative to molten metal flow through the tundish. These dams extend upwardly from the bottom of the tundish. It also is known to mount one or more weirs or baffles within the inside of a tundish with the weirs or baffles extending in a transverse direction relative to the molten metal flow through the tundish. These weirs or baffles are attached to the spaced side walls forming the tundish and extend vertically downwardly from the top of the tundish to a point a short distance above the inside bottom of the tundish. Metal turbulence is mitigated and gas bubbles and oxide inclusions may be removed to liquid slag floating on the upper surface of the molten metal as the molten metal flows along a tortuous path along the bottom of the tundish over the tops of dams and/or under the weirs while flowing from a ladle through the tundish to a casting mold. Typical weirs and dams are illustrated in U.S. Pat. No. 4,671,499. It also is known to pass molten metal through porous ceramic filters for removing oxide inclusions. U.S. Pat. No. 5,511,766 illustrates the use of dams and baffles having porous refractory filtering elements for removing inclusions as molten metal passes through a tundish.
It also is known to modify the bottom of a tundish to include an upwardly inclined surface or ramp to elevate molten metal flow within a tundish. U.S. Pat. No. 5,131,635 discloses a tundish impact pad having rising flow channels for reducing turbulence and impurities. This patent illustrates ramp-like flow channels that are inclined upwardly causing molten steel to rise as the steel flows through the tundish toward an exit port. British patent 2,164,281 relates to an inside surface of a tundish bottom modified to include a replaceable porous gas injection tile for separating inclusions. The tile encapsulates an inert gas conduit extending between each of the spaced sides of the tundish. The tile includes a porous ramp-like downwardly inclined surface for deflecting molten metal upwardly in a direction toward an exit port within the tundish.
Significant improvements have been made over the years in continuous casting tundish design for minimizing metal turbulence and improving metal cleanliness. Nevertheless, there remains a need for an improved tundish design because oxide clogging of a submerged entry nozzle continues to be a problem, especially reoxidation of the molten metal during the initial filling of a newly relined tundish. As a relined tundish is being filled, molten steel cascades over dams, resulting in significant reoxidation and gas bubble/slag entrainment. This amount of reoxidation at the beginning of a cast can cause clogging in the submerged entry nozzle during steady state continuous casting.